Pressurized fluidized bed reactors are known, such as shown in U.S. Pat. No. 4,869,207. In those reactors, a pressure vessel containing the reactor chamber is kept at super atmospheric pressure, that is a pressure of 2 bar or more, and preferably at a pressure of about 8-16 bar (for the combustor), although the pressure varies substantially from one installation to another, or within an installation during the operation. A very significant cost of such pressurized reactors, however, is the pressure vessel itself. As the volume of the pressure vessel increases, the costs escalate in a geometric rather than linear manner. Reducing the sizes of components, rearranging components, or eliminating the need for components, can therefore have a dramatic effect on the cost of the vessel and the competitiveness of the process. Therefore, it is desirable to maintain the pressure vessel at minimum size. One component that consumes significant space within the vessel is the conventional hot cyclone. When a conventional cyclone separator is utilized with the reactor chamber within the pressure vessel there is significant wasted space, and the pressure vessel must be made proportionately larger in order to accommodate a conventional cyclone. If the cyclone is placed outside the main pressure vessel which contains the reactor chamber, then a separate pressure vessel must be provided. Also, conduits and seals leading the hot flue gases from the reactor chamber to the externally located cyclone separator must be provided, as well as recirculating conduit between the externally located cyclone separator and the reactor vessel. These would further increase the costs and also make the maintenance more complicated.
In an atmospheric circulating fluidized bed reactor with a cyclone separator that is distinctly non-circular, and typically having a quadrate cross-section of the vortex chamber or gas separator therein. In this kind of system for atmospheric combustion of fuel material, the geometry is for atmospheric applications and the design of the geometry is ruled mainly by manufacturing and/or erection costs and adaptation of cooled cyclone to the cooled reactor walls. In contrast, for pressurized applications of a circulating fluidized bed reactor, other distinct benefits can be achieved by design of the geometry of the reactor which advantages would not be considered important and/or would not apply for atmospheric applications.
As can be seen for example from U.S. Pat. No. 4,793,292, in pressurized fluidized bed applications it has commonly been considered essential to utilize the gas space of the pressure vessel as efficiently as possible. That demand has previously been tried to be accomplished by forming the reactor cross section to follow the circular form of the pressure vessel as strictly as possibly. For example, U.S. Pat. No. 4,593,292 shows several modifications of reactor cross section and even a reactor alternative (including a separator) made of plurality planar walls, in order to approach a circularly formed cross section.
In atmospheric reactors there are no special requirements to the form of the cross section. However, it has surprisingly been found according to the present invention that when the distinctly non-circular cyclone separator, or a plurality thereof, are provided in association with a pressurized fluidized bed reactor, contrary to common practice using circular cyclone separators, the cross-over duct to the circular cyclone separator can be eliminated as the non-circular cyclone can be located adjacent to the reactor. This elimination of the cross-over duct yields a much more compact arrangement and allows a minimum sized pressure vessel, and thus results in economic construction of a pressurized fluidized bed reactor. There is also no need to locate a cyclone external to the pressure vessel in a separate auxiliary pressure vessel, since the compact arrangement of the furnace and non-circular cyclones can be conveniently arranged to closely fit within a circular space.
The compact arrangement of the cyclone separator in the pressurized fluidized bed reactor according to the present invention has still other advantages. Firstly, because of the compact nature and arrangement thereof, there is room for other structures, for example allowing ceramic filter elements, such as ceramic candle or honeycomb filters, to be mounted in the same pressure vessel as the reactor chamber and the cyclone separator (e.g. below or above the cyclone separator). Thus, a second pressure vessel is not necessarily needed to be provided for gas filtration, thereby reducing the costs of a complete system substantially. Also, the compact arrangement results in other advantages for the pressurized combined-cycle system. The compact structure of, not only the cyclone separator, but the whole Pressurized Circulating Fluidized Bed (PCFB) reactor, results in optimized space utilization of the pressure vessel. In a combined cycle process the operation of a gas turbine and steam turbine is combined, and therefore the steam generation circuit is not similar to that of Atmospheric Circulating Fluidized Bed (ACFB) combustion process. In combined cycle process the gas from the PCFB reactor may not preferably be cooled (contrary to ACFB where the gases should be cooled) in order to maintain gas turbine cycle efficiency at acceptable level.
Secondly, because of the compact nature and arrangement thereof, there is room to locate a bubbling Compact fluidized bed HEat exchanger (CHEX) below the non-circular cyclone. This heat exchanger can be arranged such that it has a similar cross-sectional profile to the cyclone which is above it, and it can receive hot solids from either the downcomer of the non-circular hot cyclone, or from the downflow of solids along the internal reactor walls. Heat transfer surfaces integrated into this heat exchanger can then be used to provide heated fluids used in the combined cycle power plant.
Thirdly, the cycle can also benefit from the use of high pressure steam, such as supercritical and ultra-supercritical conditions, to arranging heat transfer surfaces within the reactor using omega panels, or within the bubbling bed CHEX. This ability allows for the achievement and control of steam conditions and steam temperatures, and takes advantage of the differing heat transfer characteristics of the reactor and the CHEX. In such supercritical or ultra-supercritical steam applications a once-through steam cycle could be used and steam separator, rather than a conventional steam drum which supports natural circulation, would be applied.
According to an aspect of the present invention, a combined cycle power plant is provided, comprising the following elements: a gas compressor means for providing pressurized gas at pressure greater than 2 bar; a gas turbine means for driving the gas compressor means; a pressure vessel, circular in cross-section, and capable of withstanding pressures greater than 2 bar, and having a top and a bottom section; a pressurized circulating fluidized bed reactor enclosed by the pressure vessel, the circulating fluidized bed reactor having a reactor chamber including substantially planar walls; means for conveying the pressurized gas into said pressure vessel; means for feeding fuel into said reactor chamber; means for leading hot combustion gases away from said reactor; a centrifugal separator disposed within said pressure vessel, and having an inlet connected to said means for leading hot combustion gases away from said reactor chamber, a gas outlet leading from said separator out of said pressure vessel to the gas turbine means for expansion therein, and a return duct for recirculating separated solid particles from said separator to said reactor chamber; said centrifugal separator comprising a vertical vortex chamber having distinctly planar walls defining an interior gas space, said gas space having a cross section that is distinctly non-circular, having a circularity greater than or equal to 1.15; and a bubbling fluidized bed heat exchanger chamber in communication with said reactor chamber, said fluidized bed heat exchanger chamber having common wall sections with both the substantially planar side walls of the reactor chamber and the distinctly planar walls of the vertical vortex chamber.
According to a preferred embodiment of the present invention the combined cycle power plant includes a gas compressor providing pressurized gas (preferably air) for pressurizing the pressure vessel. The pressurized air is utilized as combustion air in the PCFB reactor enclosed by the pressure vessel. The PCFB reactor is operated as circulating fluidized bed wherein a considerable amount of solids is entrained with the gas into upper section of a reactor chamber of the PCFB reactor and further out of the reactor to the cyclone separator(s). Gas is cleaned from coarser particles (typically greater than about 25 .mu.m) in the cyclone separator(s), which separated particles are recycled back to the reactor chamber. Thus cleaned gas is passed (preferably after fine filtration) substantially uncooled into a gas turbine adapted to preferably drive the gas compressor and a generator for producing electricity. The expanded, still considerably hot gas is passed into Heat Recovery Steam Generator (HRSG) in which the heat of the expanded gas is utilized for steam generation prior to its venting into the atmosphere. The HRSG steam generation circuit is according the to present invention connected to the PCFB reactor cooling surfaces.
The PCFB reactor including the compact separator(s) according to the present invention is provided with so called membrane walls having plurality of tubes connected with each other by fins. The walls of the PCFB reactor are preferably cooled by producing steam in the tubes of the walls.
In a preferred embodiment of the present invention the PCFB reactor includes heat transfer surfaces inside the reactor chamber, preferably at least at its upper section. Such heat transfer surfaces may be so called Omega-panels which are suitable to prevailing conditions. Also according to this preferred embodiment the PCFB includes a so called Compact Heat EXchanger (CHEX) preferably at the bottom section of the reactor chamber. The CHEX is preferably connected to the reactor chamber so that it is capable of receiving solid material directly from the reactor chamber, naturally also material separated from the gas in the cyclone separator(s) may be passed into the CHEX. The CHEX is preferably a bubbling bed fluidized bed heat exchanger including heat transfer surfaces immersed in the fluidized bed of the solid material therein.
The CHEX, as well as the non-circular compact separator(s) is provided according to the present invention in connection with the PCFB reactor by utilizing a common steam generation membrane walls of the PCFB reactor. Also, the common steam generation circuit includes preferably sections in each of the above components. In this manner e.g. the thermal expansion is substantially similar to each of these components resulting in a reliable PCFB system. According to the present invention a most efficient and flexible combined cycle power plant as well as process is provided by combining the HRSG steam generation system with the steam generation system of the PCFB reactor, particularly the heat transfer surfaces inside the reactor chamber i.e. the Omega-panels and the heat transfer surfaces immersed in the fluidized bed of the solid material of the CHEX. This manner the steam generation, super heating and/or reheating of steam may be accomplished by suitably utilizing the whole system. Surprisingly it has been found that particularly a PCFB reactor according to the present invention provides an unparalleled system for combined cycle process. The present invention makes it possible to enclose substantially all required components sections of the PCFB reactor, including a reactor chamber, solid separator(s) and fluidized bed heat exchanger (CHEX), in a single pressure vessel, with optimized utilization of the space within said pressure vessel.
According to a preferred embodiment of the present invention the steam cycle of the combined cycle process comprises steam generation membrane walls of the PCFB reactor, steam generation heat transfer surfaces in the HRSG flue gas pass, steam superheating and/or reheating heat transfer surfaces in the CHEX and in the reactor chamber of the PCFB reactor. The steam generated in the steam generation walls and surfaces is superheated prior to its feeding into a steam turbine for electricity generation. In case of several e.g. two stage turbine is used the steam from the first (intermediate) stage is preferably reheated before feeding into the second (subsequent/final) stage. The present invention makes it possible to arrange adequate superheating and reheating heat transfer surfaces in connection with the PCFB reactor in a substantially small diameter pressure vessel, by providing a compact PCFB reactor.
Advantageously the superheating of steam is controlled according to the present invention by optionally directing superheated or reheated steam to the heat transfer surface in the reactor chamber of the PCFB and to the heat transfer surfaces in the CHEX, or vice versa depending on the design conditions and operating load of the power plant. At low load conditions the amount of heat removed in the CHEX can be reduced or eliminated by reducing air flow to the CHEX and by bypassing the flow of solids through it. Therefore according to the present invention the solids may be passed into the CHEX, or may bypass the CHEX, for superheating and/or reheating of the steam. Due to the compact structure of the PCFB reactor all necessary devices are possible to arrange in a single pressure vessel.
The centrifugal separator disposed within the pressure vessel has an inlet connected to a means for leading hot combustion gases away from the reactor chamber, a gas outlet leading from the separator to the next processing phase, which usually is a ceramic filter or other particulate removal device located inside or outside of the vessel, and ultimately out of the pressure vessel to depressurization in the gas turbine and low level heat recovery. A solids return duct for recirculating separated solid particles from the separator to the CHEX, or directly to the reactor chamber, is also provided. The centrifugal separator comprises a vertical vortex chamber having distinctly non-cylindrical walls defining an interior gas space, the gas space having a cross section that is distinctly noncircular, having a circularity greater than or equal to 1.15. The gas space typically has a quadrate cross section, the cyclone separator made from substantially flat panels.
The centrifugal separator may comprise a first centrifugal separator, and there may be a second centrifugal separator having the same basic components, as described above, as the first separator. Separators may be disposed on opposite sides of the reactor chamber, connected to the reactor chamber side walls, or may be disposed on the same side of the reactor chamber positioned next to each other or one above the other. If they are positioned one above the other, and if one separator gas outlet discharges upwardly, the other (the upper separator) preferably discharges downwardly so that there is a common plenum connected to the gas outlets. In order to optimize the arrangement of the reactor, non-circular cyclones, CHEXs, and potentially ceramic filters within the vessel, multiple substantially identical separators may be provided mounted in groups (e.g. pairs) on opposite sides of the reactor chamber. The reactor chamber may have a first cross-sectional area, and each of the separators has a second cross-sectional area of the gas space thereof, and those cross-sectional areas may be substantially equal.
The gas compressor means for pressurizing the pressure vessel may comprise means for introducing oxygen containing gas under pressure to the vessel to pressurize the interior of, the pressurizing gas flow also comprising means for supplying fluidizing gas to the reactor chamber at the bottom thereof. The omega panels may be provided in the reactor chamber extending along the length thereof and the separators may be mounted on the lengthways sides of the reactor chamber, parallel to the omega panels.
The reactor may further comprise a plurality of ceramic filtering means such as candle, monolithic, or honeycomb filters mounted in a support structure within the pressure vessel, and having a dirty gas inlet, a clean gas outlet, and an ash outlet; the dirty gas inlet connected to the separator gas outlet. The term "ceramic filtering means" as used in the specification and claims means conventional ceramic candle, monolith, or honeycomb filters, or improved filters developed in the future, capable of filtering particles out of high temperature gases such as flue gases from fluidized bed reactors. A number of different arrangements may be utilized to accommodate the ceramic filtering means. In one arrangement, the separator is mounted along a side of the reactor chamber connected to a side wall thereof, and the gas outlet is directed downwardly, and the support structure and the ceramic filtering means filters are mounted to the same side wall of the reactor chamber as the separator, beneath the separator, the filters of the filtering means extending generally horizontally.
According to still another aspect of the present invention a combined cycle power plant is provided, the plant comprising a air compressor providing pressurized air at pressure greater than 2 bar; a gas turbine means for driving the gas compressor means; a pressure vessel, circular in cross-section, connected to said air compressor and being capable of withstanding pressures greater than 2 bar; a pressurized circulating fluidized bed reactor enclosed by the pressure vessel, the circulating fluidized bed reactor having a reactor chamber, rectangular in cross-section, including substantially planar steam generation tube walls having a bottom section; means for leading hot combustion gases away from said reactor; a centrifugal separator disposed within said pressure vessel being adapted to the reactor chamber for receiving and purifying hot combustion gases, having a gas outlet leading from said separator out of said pressure vessel; said centrifugal separator comprising a vertical vortex chamber having distinctly planar steam generation tube walls defining an interior gas space; and a CHEX bubbling fluidized bed heat exchanger chamber having distinctly planar steam generation tube walls defining an interior of said chamber, said chamber being connected to the bottom section of said reactor chamber; a heat recovery unit adapted to the gas turbine means for recovering heat from gas discharged therefrom; a steam generation cycle having a steam turbine, steam generation surfaces including said steam generation walls, and steam superheating surfaces.
It is the primary object of the present invention to provide a combined cycle power plant with a pressurized circulating fluidized bed reactor with cyclone separator(s), as well as compact, integrated bubbling bed fluidized bed heat exchanger(s) within the pressure vessel. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.